Optical transmission systems and amplifier control apparatuses and methods

ABSTRACT

Optical systems of the present invention generally include an optical signal controller disposed along an optical link between two optical nodes. The optical signal controller is configured to provide a monitoring signal from an optical signal passing between the nodes as a plurality of wavelength sub-bands at least one of which includes a plurality of signal channels. The controller generates a compensating channel having an optical power that is a function of the monitoring signal power in the plurality of wavelength sub-bands or total power. The compensating channel is combined with the optical signal to compensate for power variations in the optical signal passing between the nodes. In addition, the compensating channels can be used to transmit communication or system supervisory information between monitoring points and/or nodes in the system.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 10/936,074, filed Sep. 8, 2004, which is a continuation of U.S.patent application Ser. No. 10/390,378, filed Mar. 17, 2003, nowabandoned, which is a continuation of U.S. patent application Ser. No.09/317,141, filed May 21, 1999, now U.S. Pat. No. 6,563,614.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

BACKGROUND OF THE INVENTION

The present invention is directed generally to optical transmissionsystems. More particularly, the invention relates to controlling opticalsignal characteristics in optical links including links containingoptical amplifiers, such as erbium doped fiber amplifiers (“EDFAs”).

Digital technology has provided electronic access to vast amounts ofinformation. The increased access has driven demand for faster andhigher capacity electronic information processing equipment (computers)and transmission networks and systems to link the processing equipment.

In response to this demand, communications service providers have turnedto optical communication systems, which have the capability to providesubstantially larger information bandwidth transmission capacities thantraditional electrical communication systems. Information can betransported through optical systems in audio, video, data, or othersignal formats analogous to electrical systems. Likewise, opticalsystems can be used in telephone, cable television, LAN, WAN, and MANsystems, as well as other communication systems.

Early optical transmission systems, known as space division multiplex(SDM) systems, transmitted one information signal using a singlewavelength in separate waveguides, i.e. fiber optic strand. Thetransmission capacity of optical systems was increased by time divisionmultiplexing (TDM) multiple low bit rate, information signals into ahigher bit rate signals that can be transported on a single opticalwavelength. The low bit rate information carried by the TDM opticalsignal can then be separated from the higher bit rate signal followingtransmission through the optical system.

The continued growth in traditional communications systems and theemergence of the Internet as a means for accessing data has furtheraccelerated the demand for higher capacity communications networks.Telecommunications service providers, in particular, have looked towavelength division multiplexing (WDM) to further increase the capacityof their existing systems.

In WDM transmission systems, pluralities of distinct TDM or SDMinformation signals are carried using electromagnetic waves havingdifferent wavelengths in the optical spectrum, i.e., far-UV tofar-infrared. The pluralities of information carrying wavelengths arecombined into a multiple wavelength WDM optical signal that istransmitted in a single waveguide. In this manner, WDM systems canincrease the transmission capacity of existing SDM/TDM systems by afactor equal to the number of wavelengths used in the WDM system.

Optical WDM systems were not initially deployed, in part, because of thehigh cost of electrical signal regeneration/amplification equipmentrequired to compensate for signal attenuation for each opticalwavelength throughout the system. The development of the erbium dopedfiber optical amplifier (EDFA) provided a cost effective means tooptically regenerate attenuated optical signal wavelengths in the 1550nm range. In addition, the 1550 nm signal wavelength range coincideswith a low loss transmission window in silica based optical fibers,which allowed EDFAs to be spaced further apart than conventionalelectrical regenerators.

The use of EDFAs essentially eliminated the need for, and the associatedcosts of, electrical signal regeneration/amplification equipment tocompensate for signal attenuation in many systems. The dramaticreduction in the number of electrical regenerators in the systems, madethe installation of WDM systems in the remaining electrical regeneratorsa cost effective means to increase optical network capacity.

EDFAs have proven to be a versatile, dependable, and cost effectiveoptical amplifier in optical transmission system. EDFAs can amplifyoptical signals over a wavelength range spanning from approximately 1500nm to 1600 nm. In addition, the amplification is polarizationindependent and introduces only low levels of channel to channelcrosstalk.

However, the characteristics that make EDFAs so useful, also have somenegative side effects. For example, because EDFAs provide gain over awavelength range of the WDM signal, the amplification of the eachchannel varies with the power of the channel, as well as the total WDMsignal power. Therefore, if a channel is added or dropped or a channelhas a power variation, all of the channels will experience a gainvariation that adversely affects the signal quality.

In addition, EDFAs do not equally amplify each channel within thewavelength range. Thus, when channels are added or dropped or a channelhas a power variation, the remaining channels will not only incur gainvariations, but the gain variations will generally be nonuniformlydistributed across the remaining channels.

The signal degradation resulting from nonuniform gain variations acrossthe wavelength range is compounded in systems having cascaded EDFAs aswould be expected. The gain variations, especially in cascaded amplifierchains, can introduce system instability and noise that results insignal distortion, attenuation, and/or loss, and greatly diminish WDMsystem performance.

Automatic gain control (“AGC”) and automatic power control (“APC”)techniques have been developed to compensate for or suppress channelgain variations in EDFAs. AGC and APC schemes for controlling amplifiersare generally-similar in operation owing to the amplifier relationshipthat Power_(OUT)/Power_(IN)=Gain.

AGC and APC, schemes can generally be categorized as feedback orfeed-forward amplifier control schemes depending upon whether the signalis monitored after passing through the amplifier or before entering theamplifier. A general description of AGC and APC schemes can be found in“Erbium-Doped Fiber Amplifiers, Principles and Applications” by EmmanuelDesurvire (1994), pp. 469-480 (“EDFA94”), which is incorporated hereinby reference. A brief, more recent summary is provided in “DynamicEffects in Optically Amplified Networks”, Optical Amplifiers and theirApplications (“OAA”) Jul. 21-23, 1997, MC4-1-4, (“OAA97-1”)

Amplifier control in either scheme is generally achieved by one of twomethods. The first method is to control the amplifier gain or power byvarying the amplifier pump power in response to the monitored signal,such as described in U.S. Pat. Nos. 4,963,832 and 5,117,196. The secondmethod is to introduce a compensating, or control, signal to control theamplifier gain or power, such as described in U.S. Pat. No. 5,088,095and “Dynamic Gain Compensation in Saturated Erbium-Doped Amplifiers”,IEEE Photonics Technology Letters, v3, n5, pp. 453-455 (1991)(“PT91-1”).

Feedback control can be based on monitoring one or more signal channelsor pilot tones, and/or optical noise at the exit of the amplifier, asdescribed in the above-referenced documents. Further examples of pilottone monitoring can be found in Electronics Letters, Sep. 14, 1989, v25,n19, pp. 1278-1280, (“EL89-1”) and total optical power monitoring can befound more recently in OAA Jul. 11-13, 1996, PDP4-1-5 (“OAA96-1”). InU.S. Pat. No. 5,506,724, ASE associated with a counter-propagatingcompensating/control channel is monitored to provide feedback controlover the control channel.

All optical gain control methods are described in Electronics Letters,Mar. 28, 1991, v27, n7, pp. 560-1, (“EL91-1”) and U.S. Pat. No.5,239,607. The all optical AGC schemes couple amplified spontaneousemission (“ASE”) from the amplifier through a feedback loop, which isinjected into the amplifier input to form a ring laser. The formation ofthe ring laser locks the gain of the amplifier independent of the inputpower of the signal at other wavelengths.

Feedback schemes are generally desirable, because the schemes can alsoaccount for changes that occur in amplifier performance over time, aswell as the input power changes. See “Automatic Gain Control in CascadedErbium Doped Fibre Amplifier Systems”, Electronics Letters, Jan. 31,1991, v27, n3, pp. 193-195, (“EL91-2”).

Conversely, feed-forward schemes do not inherently account forvariations in amplifier performance. However, feed-forward schemes inamplifier chains can indirectly account for variations in precedingamplifiers, because the variations will generally evidence themselves ininput power variations in successive amplifiers.

An advantage of feed-forward schemes, as discussed in PT91-1, is thatthe schemes can be implemented without feedback from remote amplifiersites. Therefore, feed-forward control loops can be deployed atlogistically convenient locations in a network and operatedindependently from the amplifiers, as discussed in EL91-2. Also,feed-forward schemes allow the WDM signal to be monitored before orafter control channels are combined with the optical signals.

As described in EDFA94 (pages 475-6), it is desirable to control inputsignal variations at optical switching nodes in optical networks toequalize signals originating from different stations. Either feedback orfeed-forward control can be provided to control the signal input power.For example, see Optical Fiber Communication (“OFC”) ConferenceTechnical Digest 1997 TuP4, pp. 84-5 (“OFC97-1”), 22^(nd) EuropeanConference on Optical Communications 1996 (“ECOC96”) 5.49-52 andEuropean Patent Application No. 0829981A2.

While the signal input can be equalized at each node in a network, itgenerally remains necessary to provide individual amplifier controlalong an amplifier chain to account for amplifier performancevariations. In this regard, EDFA94 (page 472) cautions that“cancellation of transient saturation is achieved by keeping constantnot the total EDFA input power, but the sum of all input powers weightedby their respective saturation powers”. However, the author concedesthat in WDM systems, the required spectral analysis to controlamplifiers based on balancing the amplifier saturation is not practical.

Another shortcoming of current control channel schemes is that theschemes can not be used to protect against large power variations, whichmay occur in dense WDM systems. Large increases in the control channelpower during gain transients can produce spectral hole burning in EDFAsthat can degrade the system performance to a greater extent than thegain transients itself. As such, current control channel schemes havelimited applicability in WDM systems.

In view of the expanding use of WDM systems and the desire to performoptical networking, it is becoming increasingly necessary to providemore precise and versatile amplifier control. The more highlycontrollable amplifiers and systems will help drive the furtherdevelopment of high capacity, more versatile, longer distancecommunication systems.

BRIEF SUMMARY OF THE INVENTION

The apparatuses and methods of the present invention address the aboveneed for higher performance optical systems. Optical systems of thepresent invention generally include an optical signal controllerdisposed along an optical link between two optical nodes. The opticalsignal controller is configured to provide a monitoring signal from anoptical signal passing between the nodes in a plurality of wavelengthsub-bands at least one of which includes a plurality of signal channels.The optical signal controller introduces power in a plurality ofcompensating channels the intensity of which is a function of themonitoring signal power in the plurality of wavelength sub-bands or thetotal power. The compensating channels are combined with the opticalsignal to compensate for power variations in the signal channels passingbetween the nodes.

In various embodiments, the optical signal controller can be configuredto provide analog or digital control over performance variations thatoccur in one or more optical amplifiers in the link. Performance controlis achieved by monitoring the input power to the amplifier in two ormore sub-bands of the amplifier wavelength range. The optical signalcontroller then varies either the power of one or more compensatingchannels and/or the amplifier power in response to the monitoringsignals to minimize the gain variations within each sub-band.

Compensating, or control, channels can be provided to compensate forinput power/gain variations within each of the sub-bands. Thecompensating channels can be at wavelengths within or outside thewavelength range of the compensated sub-band. The compensating channelsources can be responsive to power variations in more than one sub-band.In various embodiments, the power in two or more compensating channelscan be varied to maintain an average gain in the remaining signalchannels or total optical signal power.

The compensating channels can be introduced at nodes, which includeoptical components, such as transmit and/or receive terminals, opticalrouters, switches, and add/drop devices, or at other monitoring pointsin the link including amplifier sites. Likewise, the compensatingchannels can be removed at various monitoring points and reinserted toprovide flexibility in the control of each sub-band throughout theoptical link.

In various embodiments, the compensating channels can be used to carryinformation signals between two points. For example, one or more of thecompensating channels can be used to carry communication traffic(payload) between nodes and/or monitoring points on the link. In thismanner, dedicated add/drop capacity can be provided within the linkwithout sacrificing system signal channel capacity. Similarly, one ormore of the compensating channels can be used to carry systemsupervisory information through the link directly between two points.

Optical systems of the present invention can include a plurality ofnodes and links interconnected optically and/or electrically to form anoptical network. The optical systems can also include network managementto provide monitoring, provisioning and control of various network nodesand elements, such as amplifiers, etc., wavelength allocation andprovisioning in the optical system.

The controller can generally be operated employing optical-electricalcontrol loops and all-optical loops depending upon the systemconfiguration. In various embodiments, an optical splitter is used toprovide the monitoring signal in a wavelength range of interest from theoptical signal passing through transmission fiber in the link. Inembodiments, the monitoring signal can be provided before or after theinsertion of the compensating channels at the input to an amplifier.Alternatively, the power in the sub-bands can be monitored andcontrolled based on the output of an amplifier.

The wavelength range can be partitioned into sub-bands based on the gainprofile of the optical amplifier(s) being used in the system. Eachsub-band monitoring signal can be used to control its correspondingcompensating channel source to maintain a gain profile within thesub-bands as the optical signals pass through optical amplifiers. It isgenerally desirable to partition the wavelength range into sub-bandsover which the gain profile of the optical amplifier(s) is substantiallyconstant ordoes not greatly vary. In this manner, the variation in thecompensating channel power will generally track the variation of theinput signal power in that sub-band.

Compensating channels can be used in combination with pump control tocompensate for input signal variations in the amplifiers. In someinstances, it may be desirable to provide for control in the link usingboth compensating channels to minimize input power variations and pumpcontrol. Alternatively, the monitoring signals can be used to controlthe gain of the amplifier by varying the pump power, drive current, etc.provided to the amplifier.

In an embodiment, the optical signal controller is positioned before afirst of one or more EDFAs. The wavelength range is divided into fourcontiguous sub-bands spanning the wavelength range of a WDM signal beingamplified in the link. One compensating channel at a wavelength withineach sub-band is used to compensate for gain variation within thesub-band. The compensating channels are controlled based on the powerwithin the sub-band to substantially compensate for power variationsintroduced into the optical link. The compensating channels of thepresent invention can also be used in combination with various AGC andAPC schemes at the individual amplifiers. The AGC or APC schemes can useeither the signal channels or the compensating channels to control theamplifier performance.

Thus, the apparatuses and methods of the present invention provide forcontrol of the gain profile over a range of wavelengths in opticaltransmission systems. Accordingly, the present invention addresses theaforementioned problems and provides apparatuses, methods, and opticalsystems that provide increased control over optical signalcharacteristics in the system. These advantages and others will becomeapparent from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described, by way ofexample only, with reference to the accompanying drawings for thepurpose of illustrating embodiments only and not for purposes oflimiting the same; wherein like members bear like reference numeralsand:

FIGS. 1-4 show optical systems of the present invention;

FIGS. 5-8 show controller embodiments of the present invention; and,

FIGS. 9-10 show various optical source embodiments of the presentinvention.

DESCRIPTION OF THE INVENTION

FIG. 1 shows an embodiment of an optical system 10 including an opticalcontroller 12 positioned to control one or more characteristics of anoptical signal passing between two optical processing nodes 14 in anoptical link 15. The system 10 can be embodied using one or moreserially connected point to point links (FIG. 1) or in a network, whichcan be configured in various architectures (FIG. 2) and controlled by anetwork management system 16.

The optical signal controller 12 can be disposed at various monitoringpoints along a transmission fiber 18 in the optical link 15 between twonodes 14, as shown in FIG. 2. For example, the signal controller 12 canbe selectively positioned relative to one or more optical amplifiers 20disposed along the transmission fiber 18 to control the characteristicsof the optical signal being amplified in the link 15.

It is often desirable to position the signal controller 12 at the samephysical site as one or more of the nodes 14 at the beginning of theoptical link 15. In this configuration, the optical signalcharacteristics can be controlled through the entire link from alogistically convenient location.

As shown in FIG. 3, the optical system 10 will generally include atleast one transmitter 22 for transmitting optical signals including atleast one information carrying signal wavelength λ_(i), “signalwavelength or channel”, through the optical transmission fiber 18.Furthermore, the optical system 10 will generally include at least oneoptical signal receiver 24 for receiving the optical signals from thefiber 18. Also, the controller 12 can be included near or within thenode 14 as shown in FIG. 3.

The system 10 shown in FIG. 3 can be deployed as a WDM system includinga plurality of transmitters 22 _(i) for providing a plurality ofinformation carrying wavelength λ_(n) to a plurality of receivers 24_(j). Wavelength selective or non-selective optical combiners 26 can beused to combine the optical signals produced by the transmitters 22 _(i)into a WDM optical signal that is transmitted through the transmissionfiber 18. Optical distributors 28 are provided to distribute, eitherselectively or non-selectively, the WDM signal to the receivers 24 _(j).

The transmitters 22 used in the system 10 will generally include a laseroptical source, but can include other coherent as well as suitableincoherent optical sources as appropriate. Information can be impartedto an optical carrier either by directly modulating a laser or byexternal modulating an optical carrier emitted by the laser.Alternatively, the information can be imparted to an electrical carrierthat can be upconverted onto an optical wavelength to produce theoptical signal. Similarly, the optical receiver 24 used in the presentinvention can include optical receivers known in the art employingvarious detection techniques, such coherent detection, optical filteringand direct detection, and combinations thereof. Additional versatilityin systems 10 configured as networks, such as in FIG. 3, can be providedby employing tunable transmitters 22 and receivers 24 in optical nodes14.

An embodiment of the optical signal controller 12, shown in FIG. 3,includes one or more optical compensation sources 30 for providing powerin one or more compensating, or control, channel wavelengths λ_(ci),“compensating channels”. The compensating channels are combined with theoptical signal channels λ_(i) via the combiner 26 and transmittedthrough the fiber 18 in the link 15. The controller 12 also includes anoptical distributor 28, such as a low ratio splitter/tap, to provide amonitoring signal within a wavelength range of interest from the opticalsignal passing through the transmission fiber 18.

The optical combiners 26 and distributors 28 can include wavelengthselective and non-selective (“passive”), fiber and free space devices,as well as polarization sensitive devices. Standard or WDMcouplers/splitters, circulators, dichroic devices, prisms, gratings,etc., which can be used alone or in combination with various tunable orfixed wavelength transmissive or reflective filters, such as Bragggratings 34, Fabry-Perot devices, etc. in various configurations of theoptical combiners 26 and distributors 28. Furthermore, the combiners 26and distributors 28 can include one or more stages incorporating variousdevices to multiplex, demultiplex, or broadcast signal wavelengths λ_(i)in the optical systems 10.

Source controllers 32 are configured to receive the monitoring signalwithin respective sub-bands and control the optical compensation sources30 in response to the monitoring signals. Alternatively, the sourcecontrollers 32 can be configured to control the plurality ofcompensation sources 30 in response to the total optical signal power ora combination of the sub-band and total powers.

The absolute and relative locations where the compensating channelsλ_(ci) are introduced into the fiber 18 and the monitoring signal isprovided from the fiber 18 can be varied as appropriate. The monitoringsignal can include both the compensating channels and the signalchannels to provide feedback control over the compensating channelpowers.

Likewise, the compensating channels λ_(ci) can either be removed beforeor at the end of the link 15 depending upon the system configuration. Itis generally desirable to separate and reinsert compensating channelsλ_(ci) respectively before and after combining signal channels fromdifferent links 15 to provide increased system control and flexibility.

The optical signal controller 12 can also bemused to transmitcommunication traffic (payload) and/or system supervisory informationvia the compensating channels between nodes 14 or various monitoringpoints within the link 15. When used to transmit communications traffic,transmitters 22 can generally be used as the optical compensatingsources 30. It will be appreciated that appropriate modification of thetransmitter 22 to provide for variable power in the signal carryingcompensating channels may be necessary.

FIG. 4 shows an embodiment of the optical system 10 including aplurality of optical signal controller 12 embodiments. The controllers12 can be embodied as feed-forward or feedback control schemes. Thesource controller 32 can include an optical to electrical converter,such as a wavelength selective or non-selective optical receiver 24 toreceive the monitoring signal. The optical to electrical converter canbe a photodiode 34 that provides an electrical monitoring signal to thesource controller 32, which is used to control one or more opticalcompensation sources 30 and/or optical amplifiers 20. As shown in FIG.4, the amplifier 20 can include doped, i.e., erbium, or Raman fiberamplifiers 36 supplied with pump energy supplied by a pump source 38, aswell as other optical amplifiers. The source controller 32 can beconfigured to control the pump source 38 in response to the monitoringsignal.

The optical signal controller 12 can be configured to maintain asubstantially constant optical signal power distribution in opticallinks 15. A constant optical power distribution in the links 15,facilitates substantially constant optical amplifier gain performance.Thus, the gain of the individual signal channel being transmittedthrough the link 15 can be controlled to a substantially constant value,if desired. In addition, the power of the individual signal channels canbe controlled to maintain uniform and non-uniform gain profiles over thewavelength range as may be desired.

In various embodiments, the optical signal controller 12 employs aplurality of compensating channels to maintain a substantially constantgain profile for the optical signal in the link 15. The compensatingchannel wavelengths λ_(ci) can be within or outside correspondingsub-band wavelength ranges of the optical signals being transmitted inthe link 15. For example, the wavelength range of the signal channels inthe link 15 could be divided into four adjacent sub-bands, each of whichinclude a compensating channel at a wavelength within the sub-band. Thecompensating channels can be controlled in feed-forward or feedbackschemes and can also be used in various AGC and APC schemes used tocontrol the individual amplifiers.

The optical signal controller 12 is generally configured to compensatefor power changes within the sub-bands of the optical signal before theamplifier gain profile is substantially impacted. For example, theoptical signal controller 12 responds to a sudden reduction of theoptical power within a sub-band by increasing the optical power suppliedby the optical compensating source 30 associated with that sub-band.Likewise, if an increase in optical power is detected in the sub-band,the controller 12 must decrease the optical power output supplied by thecompensating source 30 on a similar time scale.

The required response time of the controller 12 is a function of thenumber of cascaded amplifiers in the link 15. One of ordinary skill willappreciate that the required response time affects the choice ofelectronics to practice various embodiments of the invention.

FIGS. 5( a-c) show an exemplary layouts of a digital and analog sub-bandcontrol loops that can be used in the controller 12 to control thesub-band compensating channel power. Generally, the controller 12includes a sub-band demultiplexer 28 _(d) to separate the monitoringsignal into sub-band optical signals. Optical to electrical converters,such as photodiodes 34, are used to detect the sub-band optical signaland generate electrical monitoring sub-band input signals S_(i). Thesub-band source controller 32 uses the electrical monitoring inputsignals S_(i) to vary the compensating channel power provided by thesub-band compensating source 30 in response to fluctuations in thesub-band input signal power. A low loss combiner 26, such as acirculator 40 and Bragg grating 42 arrangement, can be used to combinecompensating channels with the WDM signal channel in the transmissionfiber 18.

As further shown in FIG. 5( a), the sub-band source controller 32 willgenerally include an analog to digital (“A/D”) converter 44 to providethe sub-band input signal S_(i) as a digital signal to a sub-band inputcomparator differencing circuit 46. A setpoint power SP_(i), which canbe programmable, is provided for each sub-band to the differencingcircuit 46 and a monitor input error E_(i) is generated. Thedifferencing circuit 46 can provide the absolute error calculated orapply an error threshold to the calculated error. The error thresholdcan be programmably set to minimize jitter or other noise in the signalfrom causing unnecessary, and possibly destabilizing, power variationsin the control loop.

An error accumulator circuit 48 is used to provide a digital bias drivesignal B_(i) in response to the monitor error signal E_(i) to a digitalto analog (“D/A”) converter 50 to control the compensating channel powersupplied by the sub-band optical source 30. An exemplary erroraccumulator circuit 48 for a sub-band control loop is shown in FIG. 5(b) as an overall high-pass filter arrangement. It will be appreciatedthat other filter arrangements can be used in the error accumulatorcircuit 48. The arrangement shown in FIG. 5( b) is configured to drivedetected signal channel input power variations to zero by varying theoutput power of the source 30 in response to the detected variations.The bandwidth of the high-pass filter response generally establishes thelength of time during which the control laser output level is perturbedbefore the correction occurs.

In the error accumulator circuit 48, the setpoint power input errorE_(i) is amplified using a first signal amplifier 52 ₁ and provided to asumming circuit 56, which accumulates the error. A feedback loopincluding a second amplifier 52 ₂ and an addressable memory 54 is usedto implement the necessary bias signal value storage function of theerror accumulator. The stored bias signal value is fed back to thesumming circuit 56. The bias signal can be used to directly vary theoutput power of the sources 30 and thereby the compensating channelpower. Alternatively, the output of the error accumulator circuit 48 canbe used to control an external modulator or optical attenuator to varythe compensating channel power being introduced into the transmissionfiber 18.

The response of the controller 12 shown in FIG. 5( b) can be modeledusing a sampled-time analysis, where the z-transform of the control looptransfer function can be derived as:B _(out) =B _(in) −B _(out)(G ₁/(z−G ₂));

B_(out)/B_(in)(z)=(z−G₂)/(z−(G₂−G₁)) and the frequency response isB_(out)/B_(in)(ω)=(e^(jωT)−G₂)/(e^(jωT)−(G₂−G₁)).

B_(in) and B_(out)=input and output bias signal, respectively,

G₁ & G₂=gain of amplifiers 52 ₁ and 52 ₂, respectively,

T=sample time,

ω=frequency,

z=e^(jωT) (transform of the frequency)

In the embodiments shown FIG. 5( a), a monitoring signal is removed fromthe input transmission fiber 18 _(I), after the compensatingchannelsignal has been inserted into the fiber 18. Thus, the monitoringsignal includes both the signal channels and the compensating channels.As previously discussed, the monitoring signal can be removed before thecompensating channels are introduced, so as to include only the signalchannels, if so desired. Also, the monitoring signal can be passively oractively removed from the fiber in either a wavelength selective ornon-selective manner. Typically, the monitoring signal will be removedby employing a low coupling ratio, non-wavelength selective tap couplerto split off a small percentage of the total signal.

The control loops can alternatively be implemented as an analog circuit,an example of which is shown in FIG. 5( c). The output from the sub-bandphotodetectors 34 _(s) are provided to respective analog differencingamplifiers 53 _(i), either directly or via a fixed or variableattenuator 55 _(i), which can be used to provide additional control thesub-band loop. A wideband detector photodiode 34 _(w) can be used tomeasure the total input power of the monitoring signal. The total inputpower can be compared to a total power set point in one of thedifferencing amplifiers 53 and a total input power correction providedto the sub-band differencing amplifiers 53.

Similarly, in various digital embodiments generally shown by FIG. 6, thewideband detector photodiode 34 _(w) can also be incorporated to providea wideband monitoring signal S_(w) for input power error correction. Theelectrical sub-band monitoring input signals S_(i) are summed in anothersumming circuit 56 into a composite sub-band input signal S_(c), whichis compared to the wideband monitoring signal S_(w) in anothercomparator/differencing circuit 46. An input offset signal I_(e) isgenerated by the differencing circuit 46 and sent to each of thesub-band control loops. A multiplying circuit 58 allocates the inputoffset I_(e) based on sub-band error allocation setpoints K_(i). Theinput offset I_(e) can be added to either the sub-band monitoring inputsignal S_(i) or the setpoint power input error signal E_(i) dependingupon the control loop configuration.

A mismatch between the composite sub-band signals S_(c) and the widebanddetector signal S_(w) is indicative of variations in the frequencydemultiplexer 28 _(d), the sub-band detectors 34 _(s), or the widebanddetector 34 _(w). Error allocation in the controller 12 can be performedvia numerous algorithms and statically or dynamically allocated invarious distributions depending on the controller 12 configuration. Forexample, individual setpoints K_(i) and multiplying circuits 58 can beprovided for each sub-band or a common multiplying circuit 58 can beused to equally distribute various errors in the controller 12.

The signal controller 12 can include one or more central processors 60to monitor and control the sub-band control loops. The processors 60 cancommunicate with the network management system 16 to receiveinstructions and provide performance information, such as when non-zeroinput offset I_(e) or other performance variations that occur in thecontroller 12.

Analogous to the input offset monitoring, fault monitoring andremediation can be provided using an error distribution loop, as shownin FIG. 7, to redirect sub-band gain control to other viable loops inthe event of a sub-band loop failure. In various embodiments, the setpoint input error E_(i) is compared with the set point power SP_(i) todetermine whether a sub-band control loop has failed. When a sub-bandcontrol loop failure is detected, a line switch 62 passes a failureerror signal, typically the set point value SP_(i), to another summingcircuit 56, which accumulates the failure error signals from thesub-bands. The control loop failure determination can be performed usingthe central processor 60 or a locally employed decision circuit in theline switch 62. The central processor 60 can also be used to makeappropriate modifications to the correction set points for the remainingcontrol loops depending upon the failure. In addition, the failuredetermination decision threshold can be programmably implemented toprovide flexibility in the controller 12.

Alternatively, the decision circuit 62 can compare the input set pointSP_(i) directly with the sub-band input S_(i), as in FIG. 8. A commonmultiplying circuit 58 is also shown in the embodiment of FIG. 8, inwhich the error from a failed sub-band loop is evenly distributed amongthe surviving loops.

The various errors in the control loops can be distributed using anynumber of schemes. For example, the error can be distributed among thesurviving sub-band loops to maintain the gain profile of the opticalsignal. Alternately, the cumulative failure error signals may be dividedamong the remaining loops according to the inverse proportion of thecurrent laser bias of each loop. This method would lessen theprobability that any one of the surviving sub-band loops would beoverloaded upon the failure of an adjacent sub-band loop. The centralprocessor 60 can be used to monitor the redistribution of the error andmodify the redistribution to equalize or balance the power output fromthe sub-band loops depending upon the channel profile to be maintained.

The accuracy and speed of response of the controller 12 depends on astable response from the optical compensating sources 30 to the biasdrive level. The speed and accuracy of the response will generally varyover time; therefore, recalibration of the controller 12 will mostlikely be required to maintain performance levels.

As shown in FIG. 8, one or more of the central processors 60 in thecontroller 12 can be used to oversee the operation and performcalibration of the sub-band control loops. Each control loop can includea calibration device 64 for controlling the drive signal applied to theoptical source 30. The calibration device 64 can be a digital device,such as a calibration table, or an analog device, such as a linearcircuit.

During calibration, a switch 66 is used to by-pass the calibrationdevice 64 and allow the central processor 60 to apply one or more testbias signal to the sub-band sources 30. As shown in FIG. 8, the positionof the switch 66 will depend on whether an analog or digital calibrationdevice 64 is implemented in the optical signal controller 12. The errordistribution loop and fault control techniques previously described willadjust the compensating channel sources 30 that remain in operation tocompensate for the variation in the compensating channel from the source30 being calibrated.

The optical power of the source 30 can be calibrated by various knownmethods, for example by using a series of stepped bias levels. Anoptical spectrum analyzer (“OSA”) or other wavelength selective receiver68 can be used to measure the test output power of the selected source30 independently of the photodiode 34 associated with a particularsub-band. Alternatively, the test output power can be detected using thesub-band photodiode 34 _(s) in the sub-band loop being calibrated. Whenused in combination, the OSA 68 can be used to calibrate the sub-bandphotodiode 34 _(s), as well as the sub-sand source 30 and othercomponents in the sub-band control loop.

The detected test output power from either the OSA 68 or sub-bandphotodiode 34 is fed back to the central processor 60 forcharacterization of the source 30 being calibrated. The characterizationcan be used to develop a new calibration table for the source 30, whichcan be implemented when the source 30 is brought back on-line. In thismanner, each source 30 in the controller 12 can be calibrated withoutremoving the controller 12 from on-line operation. While it is possibleto configure the controller 12 to perform simultaneous multiplecalibrations, it is generally not desirable given the power distributionthat may be necessary in the remaining operational sub-band controlloops.

In embodiments exemplified in FIG. 8, the OSA 68 is positioned tocalibrate the sources 30 based on the sub-band signal in thetransmission fiber 18. It will be appreciated that the OSA 68 can beincluded within the controller 12 to more specifically calibrate theoptical sources 30. In some embodiments, it may be possible tosubstitute the OSA 68 for the wideband photodiode 34 _(w) used todetermine the sub-band input power offset.

The optical sources 30 used in the present invention can be conventionaldiode lasers as known in the art. As previously discussed, communicationtraffic or system supervisory information can be sent using thecompensating channel, if an appropriate transmitter 22 is used as thesource 30.

The optical source 30 used in the controller 12 generally have to becapable of operation over a wide power range to maintain the opticalsignal gain profile upon the failure of one or more sub-band controlloops. The bandwidth of the source 30 must therefore be sufficientlybroad to prevent Stimulated Brillouin Scattering (“SBS”) during highpower operation and sufficiently narrow not to interfere with adjacentsignal channels. Broad band optical sources or narrow band sources thathave been broadened, via dithering, external cavity gratings, or othertechniques, can be used as the optical sources 30.

For example, the optical source 30 can be embodied as semiconductoroptical amplifier (“SOA”), or fiber laser, 70 operated in a lasing modeand stabilized to a desired wavelength as shown in FIG. 9( a-d). The SOA70 can provide compensating channels over a wide power range and Bragggratings 42, or other reflective elements 78, can be used to control thelasing wavelength and bandwidth. For example, in FIG. 9( a), a highreflectivity Bragg grating 42 _(H) and a lower reflectivity grating 42_(L) can be written into fiber pigtails on the SOA 70 to control thelasing wavelength. In FIG. 9( b), a Bragg grating is provided on anoutput port of a passive or wavelength selective coupler 72 in a ringconfiguration with the SOA 70. In various embodiments, the reflectiveelement 78, i.e., Bragg grating 42, can be tuned to vary the outputwavelength of the SOA source 30.

As shown in FIGS. 9( c-d), the optical source 30 can include the SOA 70,or a fiber laser, that is frequency stabilized using a feedback loopincorporating a saturable absorber 74, such as an unpumped erbium fiber.In embodiments exemplified by FIG. 9( c), an external cavity is formedusing a three port circulator 40 in a feedback ring configuration and awideband reflective device 78. Course mode selection in the feedbackring can be made using a narrow pass filter 76, such as a Fabry-Perotfilter. The saturable absorber 74 serves to lock the lasing mode of theSOA 70 or fiber laser within the passband of the filter 76.

Similarly, the circulator 40 can be replaced by a wavelength selectiveor passive coupler 72 and the SOA 70, or fiber laser, can beincorporated into the ring as shown in FIG. 9( d). Furthermore, anisolator 80 can be employed to provide a unidirectional propagation inthe ring embodiments.

Course wavelength selection in saturable absorber embodiments willgenerally provide one or more lasing modes within the desired wavelengthrange, one of which will tend to become the dominant mode. As thedominant mode or modes emerge, the saturable absorber 74 will act toprevent mode hopping because of the high loss associated with modesoutside the saturating dominant modes. Also, an external frequencysource can be used in place of the narrow band filter 76 to select thesaturating mode/frequency emitted by the source 30. The embodiments ofFIGS. 9( a-d) have been described with respect to providing compensatingchannels, but can also be deployed as fixed or tunable wavelengthoptical sources in optical transmitters, local oscillators, and otheroptical source applications.

In another embodiment, the compensating channel provided by the opticalsource 30 can be broadened using embodiments shown in FIGS. 10( a&b).For example, the compensating channel can be phase modulated to maintainthe compensating channel power, while broadening the signal by creatingsidebands (FIG. 10( a)). A phase modulator 82 can be driven at multiplefrequencies to control SBS in fibers having different core sizes andsusceptibilities to SBS. The wavelength of the source 30, such asprovided by a DFB laser, can be controlled using frequency lockingdevices 84 and schemes as is known in the art. Likewise, narrow bandreflective devices 78, such as narrow band Bragg gratings 42, can beemployed to further control the wavelength of the laser.

Similarly, the optical source 30, such as a DFB laser, can be ditheredor modulated directly to broaden the linewidth of the source 30, asshown in FIG. 10( b). The amplitude of the dithering or modulating ofthe laser drive current can be varied with power to further decrease thesusceptibility to SBS. As in FIG. 10( a) embodiments, the wavelength ofthe optical source 30 can be controlled using various narrow bandwavelength selective devices 78, such as Bragg gratings.

Those of ordinary skill in the art will appreciate that numerousmodifications and variations that can be made to specific aspects of thepresent invention without departing from the scope of the presentinvention. It is intended that the foregoing specification and thefollowing claims cover such modifications and variations.

1. An apparatus comprising: an optical signal controller configured toprovide a monitoring signal from an optical signal in a plurality ofsub-band signals at least one of which includes a plurality of opticalsignal channels, and control the characteristics of the optical signalbased on the sub-band signal characteristics, wherein said opticalsignal controller is configured to combine a compensating channel havinga power based on the sub-band signal powers with the optical signalchannels for transmission of both the compensating channel and theoptical signal channels through at least two optical amplifiers tocontrol the characteristics of the optical signal channels passingthrough the at least two optical amplifiers via a combined compensatingchannel and optical signal channels.
 2. The apparatus of claim 1,wherein said controller includes a plurality of compensating channelsources providing compensating channels having powers based on thesub-band signal powers.
 3. The apparatus of claim 2, wherein at leastone of said plurality of compensating channel sources is configured tocompensate for power variations in said other compensating sources. 4.The apparatus of claim 1, wherein said controller includes at least onecompensating channel source for each of the plurality of sub-bands. 5.The apparatus of claim 4, wherein said controller includes onecompensating channel source for each sub-band and each of saidcompensating sources provides a sub-band compensating channel at awavelength within the sub-band and the power of the sub-bandcompensating channel is controlled to maintain an optical signal gainprofile within the sub-bands.
 6. An optical system comprising: at leasttwo optical nodes configured to pass an optical signal via an opticaltransmission fiber between said at least two nodes; and, an opticalsignal controller configured to provide a portion of the optical signalas a monitoring signal in a plurality of sub-band signals at least oneof which includes a plurality of optical signal channels, and controlthe characteristics of the optical signal based on the sib-band signalcharacteristics; wherein said optical signal controller is configured tocombine a compensating channel having a power based on the sub-bandsignal powers with the optical signal channels for transmission of boththe compensating channel and the optical signal channels through atleast two optical amplifiers to control the characteristics of theoptical signal channels passing through the at least two opticalamplifiers via a combined compensating channel and optical signalchannels.
 7. The optical system of claim 6, wherein: said optical signalcontroller is configured to separate a portion of the optical signalinto a plurality of wavelength sub-band signals and combine acompensating channel having a power sufficient to maintain a gainprofile in the optical signal amplified by at least one of the opticalamplifiers.
 8. The optical system of claim 7, wherein said signalcontroller is configured to receive a feedback signal from at least oneof the optical amplifiers and control the power of at least one of thecompensating channels.
 9. The optical system of claim 6, wherein: saidoptical signal controller is configured to separate a portion of theoptical signal into a plurality of sub-band signals and control the gainprofile of the optical signal being amplified by at least one of theoptical amplifiers based on the sub-band signal powers.
 10. The opticalsystem of claim 9, wherein said optical signal controller controls theamplification of the optical signals by varying the optical energysupplied by a pump source to said at least one amplifier based on thesub-band signal powers.
 11. The optical system of claim 6, wherein: saidat least one optical amplifier is configured to amplify the opticalsignal within a wavelength range; said controller is configured tocontrol variations in the amplification of the optical signal over thewavelength range by varying the compensating channel powers based on thesub-band signal powers.
 12. The optical system of claim 11, wherein saidcontroller is configured to maintain a gain profile in the opticalsignal over the wavelength range.
 13. The optical system of claim 11,wherein said controller is configured to maintain a gain profile in theoptical signal in the sub-band wavelength range by varying the power ofa compensating channel at a wavelength within the sub-band wavelengthrange.
 14. The optical system of claim 6, wherein: a first of saidoptical nodes includes at least an optical transmitter configured totransmit the optical signal in the optical transmission fiber; a secondof said optical nodes includes at least an optical receiver configuredto receive the optical signal transmitted in said optical transmissionfiber; and, at least one of the optical amplifiers disposed along saidtransmission fiber is configured to amplify the optical signal within awavelength range; and, said optical signal controller is configured tocombine a compensating channel having a power to maintain a constantamplification of the optical signal by said amplifier.
 15. The opticalsystem of claim 6, wherein: said system includes a network managementsystem in communication with said signal controller; and, said signalcontroller includes at least one central processor configured to controlsaid sub-band control loops and communicate with said network managementsystem.
 16. The optical system of claim 6, wherein said nodes includeoptical components selected from the group consisting of opticaltransmitters, optical receivers, optical routers, optical add/dropdevices, optical switches, and combinations thereof.
 17. The opticalsystem of claim 6, wherein said signal controller provides acompensating channel carrying at least one of communication signals andsystem supervisory signals.
 18. The optical system of claim 17, whereinsaid controller provides a compensating channel carrying supervisorysignals between said nodes.
 19. A method of controlling optical signalcharacteristics comprising: providing an monitoring signal from anoptical signal; separating the monitoring signal into a plurality ofsub-band signals, one of which includes a plurality of optical signalchannels; controlling the characteristics of the optical signal based oncharacteristics of the sub-band signals, wherein said controllingincludes: generating at least one compensating channel to control atleast one characteristic of the optical signal channels passing throughat least two optical amplifiers based on the at least one characteristicin the sub-band signals; combining the at least one compensating channelwith the optical signal channels for transmission of both thecompensating channel and the optical signal channels through the atleast two optical amplifiers.
 20. The method of claim 19, wherein: saidmethod includes amplifying the optical signal; and, said generatingincludes generating a compensating channel for each of the sub-bandsignals to maintain a constant amplification of each channel in theoptical signals being amplified.
 21. The method of claim 20, whereinsaid controlling includes clamping the gain of at least one compensatingchannel being amplified.
 22. The method of claim 19, wherein saidgenerating includes generating at least one compensating channel tomaintain a constant power in the optical signal based on the sub-bandsignal powers.
 23. The method of claim 19, wherein said generatingincludes generating at least one compensating channel for each of thesub-band signals to maintain a constant power in each sub-band of theoptical signal.